1. Field of the Invention
The present invention relates primarily to control for processes involving deposited material (such as for example molten metal spraying processes).
2. State of the Art
WO-A-96/09421 discloses a technique for spraying molten metal (particularly steel) to produce self supporting articles. In the process disclosed it is clear that for a practically realisable process, accurate control of the temperature of the sprayed metal droplets and/or the temperature of the already deposited material is important. Such considerations are also relevant to spraying of other materials and other deposition processes. Additionally other parameters for spray deposition processes require monitoring regulation and control.
The spray forming process deposits molten metal (typically from electric arc spray guns) onto a substrate (typically a ceramic substrate) to form a metal shell that accurately reproduces the topography of the ceramic substrate.
The molten metal is typically produced in the guns by direct current arcing between two oppositely charged wires made of the metal being sprayed. The arcing causes the wire tips to melt and a high-pressure inert gas stream continuously strips molten material from the arc, atomising it into a spray of droplets. The gas stream carries the droplets to the surface of the object where they are deposited. Wire is continuously fed to the arc gun to maintain the flow of sprayed metal and the amount of metal that is deposited can be adjusted by changing the feed rate of the wire. The droplet spray from the guns is scanned over the surface of the ceramic substrate by a robot in a pre-determined, repetitive manner, referred to as the “path plan”.
The guns act not only as source of material but also as source of heat because the molten droplets transfer their heat to the spray formed metal shell as they cool and solidify to build up a solid metal shell. An important feature of the process described in WO-A-96/09421 is that it relies on the metal droplets undergoing prescribed phase transformations as they cool after being deposited on the surface of the sprayform. These phase transformations offset the natural contraction of the metal as it cools, allowing the dimensional accuracy of the sprayform to be maintained. In order to ensure that the required transformations occur, accurate regulation of the thermal history of the sprayed material is necessary. One method of regulating the thermal history is to ensure that the temperature of the surface at the point where the spray was deposited passes through a given temperature at a specific time after deposition. A system for regulating the thermal history of the deposited material has been proposed that adjusts one or more parameters including the height, velocity and path of the robot and the orientation of the guns. These adjustments are made relative to nominal or reference values for these variables and the purpose of the current invention is to specify a nominal path for the robot over the sprayform that will minimise the variations in temperature over the surface.